In the typical traveling-wire EDM process, also called wire-cut EDM, an electrode wire traversing a workpiece and traveling axially therethrough between a pair of wire-positioning guide members is advanced transversely to the workpiece along a predetermined path of cut therein, thus leaving a slot cut behind the advancing wire, the slot having two side surfaces which are generally symmetrical about the trajectory of the axis of the wire and only one of which is typically significant as a contour of the cut to be imparted to the workpiece. The cut slot corresponds in width generally to the diameter of the wire and, during the erosive cutting operation, generally serves to accept the machining fluid therein which not only acts as the discharge medium but to counterbalance the electrode wire against a machining pressure developing in the machining gap. The diameter of the electrode wire needs to be as small as at most 0.5 mm (and more than 0.05 mm) to ensure a desired cutting precision.
Situations, however, arise in which a layer of stock as thin as 10 to 100 .mu.m, thus still less than the diameter of the electrode, needs to be removed from a workpiece with due precision and surface finish. Such requirements are typical where a rough-shaped contour is to be finished in a secondary cutting arrangement. In such cases, as readily recognized, there is not brought about a bilateral flanking slot as described but there results a unilateral-flanking state in which one side of the advancing wire is always open-spaced. It has been found that the unilateral-flanking mode has both advantages and disadvantages. One advantage is that the machining detritus can hardly accumulate in the cutting zone so that erosive cutting proceeds quicker. One serious disadvantage is the lack of stability in wire position. The machining pressure which develops due to erosive discharges and fluid flushing in the gap tends to move the electrode wire away from an instantaneous area of the contour being cut, in a direction obliquely behind the path of wire advance and, by reason of its inherent fluctuation, tends to force the traveling wire to undulate in a plane transverse to that area of contour, hence causes a fluctuation of the wire axis in spite of the use of the wire-positioning guide members across the cutting zone. While this fluctuation is generally small in amplitude, it becomes no longer negligible where the machining surface finish required is as fine as 0.5 .mu.Rmax in roughness.